How to narrow picture

[apple2dude]

Hi all, this is my first post here. I've got an Admiral KMC13-100 from 1978 that works like a charm. My primary use for it is with a roku, which I've built into it to make it a sort of vintage style smart tv.

The only problem is that the overscan is great enough that it cuts off maybe 1/8 the picture on each side, so for example I can't see the first column of letters when I go to search something. I've got a service manual, but can't find any reference to a way to adjust the horizontal width. Most of the low voltage h-sync circuitry is housed in a chip that I can't find anywhere online.

I thought to try putting a low value, high wattage resistor in line with the horizontal deflection coil, but haven't really tried yet. Any ideas here?